290 GENERAL GEOLOGY. 



chalky beds. It readies a maximum thickness in Iowa, so 

 far as now known, of about one hundred feet, but the 

 exposures usually show a much less thickness. 



Lithologically, this formation is almost entirely a rather 

 coarse-grained, friable, more or less ferruginous sandstone ; 

 but in a few instances thin, irregular layers of clayey mate- 

 rial are found in it. Sometimes the grains of sand of which it 

 is composed, are so slightly coherent that a spade may be 

 thrust into it by a strong man. Occasionally, layers of gravel 

 occur intercalated with the sand, which would thus form con- 

 glomerate if the pebbles were firmly cemented together. At 

 Lewis, in Cass county, the sandstone contains so much brown 

 oxyd of iron that it has a uniform dark-brown color. In 

 some other places the iron acts as a firm cementing material 

 for the grains of sand, forming hard, brown, irregular layers 

 and concretions in the softer and lighter colored portions 

 of the rock. Such portions have the color and general 

 appearance at a little distance of brown hematite ore; in 

 consequence of which, persons unacquainted with the subject 

 have mistaken them for valuable deposits of iron ore. This 

 has been particularly the case in Guthrie county where there 

 are many exposures of the Mshnabotany sandstone, and 

 where, also, the hematite seams are numerous in it. 



The pebbles contained in this sandstone, so far as yet 

 observed, are all silicious, and never calcareous, although 

 some of them appear to be so before being broken open. 

 None of them have been found to be of granitic or metamor- 

 phic origin, nor even of quartzite, but appear to have been 

 derived from the chert or hornstone nodules of the older 

 formations, as several species of their fossils, in a silicified 

 condition, have been detected among the gravel. The genera 

 Favosites, Syringopora, and Cyatliopliyllum have been 

 recognized among these pebbles of the Mshnabotany sand- 

 stone in Guthrie county, all of which were probably derived 

 from the Niagara limestone as pebbles during Cretaceous 

 age. 



Economic Value. In consequence of the general softness 



